As an indicator for showing a biological state, measurement of a glycation degree is performed with respect to glycation products of various kinds of proteins. Among them, a glycation product of hemoglobin (Hb) in a blood cell (hereinafter, also referred to as “Hb glycation product”) is considered as an important indicator in diagnoses, treatments and the like for diabetes. Examples of the Hb glycation product include a product in which glucose is bonded to an α-amino group of a β chain N-terminal amino acid (valine) residue in Hb (hereinafter, also referred to as “HbA1c”); a product in which glucose is bonded to a side chain amino group of a lysine residue in Hb (hereinafter, also referred to as “GHbLys”); a product in which glucose is bonded to a side chain amino group of an arginine residue in Hb; etc. It is known that, in particular, HbA1c reflects histories (about 1 to 2 months) of an in-vivo blood glucose level, specifically, the usual average value of blood glucose level in past. An HbA1c value (HbA1c %) normally is represented by a ratio (%) of an HbA1c amount relative to a total Hb amount (or total HbA amount), and is, for example, referred to as a glycation degree of a β chain N-terminal amino acid residue in Hb or a glycation degree of a β chain N-terminal amino group.
In recent years, the detection of postprandial hyperglycemia, which is an early stage of diabetes, is emphasized. Postprandial hyperglycemia is a symptom in which even though a blood glucose level in the fasting state is within the normal range (110 mg/dL or less), the blood glucose level increases 1 to 2 hours after a meal. Generally, 2 hours after a meal, the blood glucose level increases to 140 mg/dL or more. If this postprandial hyperglycemia is left untreated, there is a possibility of developing full-blown diabetes or a possibility of increasing a risk for complication, cerebral infarction, and myocardial infarction. Therefore, it is important to detect postprandial hyperglycemia for preventing diabetes and the like. However, with respect to a sugar tolerance test adopted as a detecting method for postprandial hyperglycemia, for example, the patients have to handle the following burdens. First, for the sugar tolerance test, patients should fast at least 8 hours (for example, at least 14 hours) before the test, and then take 75 g of starch hydrolysate (glucose) for test in the fasting state. Further, because the change of postprandial hyperglycemia is fast, a blood glucose level should be measured by collecting blood 30 minutes, 60 minutes, and 120 minutes after the test as well as before the test. Moreover, since glucose digestion due to hemocyte cell occurs in the collected blood sample, the test should be performed in an institution capable of performing a prompt measurement. From these reasons, a direct measurement of glucose in the blood for detecting postprandial hyperglycemia forces time and physical burdens on patients. A method of indirectly judging postprandial hyperglycemia rather than a direct method is desired.
However, although HbA1c is a very effective indicator in diagnoses for diabetes, since HbA1c reflects an average value of blood glucose level in the past 1 to 2 months after all, postprandial hyperglycemia could not be detected from HbA1c as an indicator. This may be because of the following reasons. Specifically, in short-time rise in blood sugar, unstable HbA1c that is a precursor of the HbA1c was generated promptly. However, it is difficult to generate stable HbA1c from the unstable HbA1c. Therefore, it is considered that, on the basis of measurement of the stable HbA1c, although chronic diabetes can be judged, postprandial hyperglycemia, which shows hyperglycemia only after a meal, cannot be judged (Nonpatent Document 1). More specifically, the stable HbA1c shows the usual average value of blood glucose level in past and does not reflect intraday changes in blood glucose level affected by meals. Therefore, the stable HbA1c cannot be an indicator to grasp postprandial hyperglycemia. Further, with respect to the unstable HbA1c, since the measurement itself is difficult, it is not practical to judge postprandial hyperglycemia on the basis of the measurement of the unstable HbA1c.
[Nonpatent Document 1] Journal of Laboratory and Clinical Medicine, Vol 147, 1, 1-2006, p21-26